Thin aluminum pigments having a narrow thickness distribution, method for producing same, and use of aluminum pigments

ABSTRACT

The invention relates to aluminum pigments which are at least partially coated with lubricant, wherein the aluminum pigments have a relative breadth of thickness distribution Δh of from 30% to less than 70%, as determined by a scanning electron microscope thickness count and as calculated on the basis of the corresponding cumulative breakthrough curve of the relative frequencies of occurrence, according to the formula Δh=100×(h 90 −h 10 )/h 50 , and an X-ray diffractogram, measured on pigments in substantially plane-parallel orientation, having one or two main peaks which do not correspond to the [111] reflexes. 
     The invention further relates to a method for the production of said aluminum pigments and to uses thereof and also to nail varnishes and printing inks containing said aluminum pigments of the invention.

The present invention relates to thin platelet-like aluminum pigmentshaving a narrow thickness distribution and to a method for theproduction thereof. The invention also relates to uses of these aluminumpigments.

Platelet-like aluminum pigments are effect pigments and aredistinguished by their unique metallic appearance and their highcovering power. On account of the platelet-like structure of theseeffect pigments, they undergo orientation in the coating medium so as tobe parallel to the substrate and cause a metallic effect due to theformation of a combination of many discrete tiny mirrors. This metalliceffect is very strongly pronounced, particularly in wet lacquers. In thecase of full-tone lacquers, there is a brightness effect dependent onthe angle of observation and/or angle of incidence, which is alsoreferred to as “flop”. Good flop is influenced by many properties of thepigments. Thus their orientation, their size and size distribution,their surface texture (roughness) and the edge texture all play animportant part.

The determining factor for a plane-parallel orientation of the pigments,which are also referred to as flakes, is not only surface chemistryincompatibilities of the aluminum pigments and the binder system butalso and especially the form factor of the pigments. The form factor isunderstood as meaning the ratio of the length d to the thickness h ofthe pigments. The length is mainly measured by laser scattering methods.In this case, the d₅₀ index of the cumulative breakthrough curve isnormally used.

Since the length of the aluminum pigments is highly dependent on theintended method of application, a high form factor and thus the bestpossible orientation can be particularly well achieved by varying thethickness of the pigments. Thin pigments undergo better orientation andtherefore have higher flop. A further important characteristic ofmetallic coatings or printing inks is their high gloss. Gloss, interalia, is a physiologically and psychologically related variable, butaccording to DIN 67 530, the “gloss power” of a plane surface isrecorded by reflectometer values. The reflection is measured at thegloss angle relatively to a standard (as a rule a black mirror glassplate).

According to this DIN standard, highly glossy specimens (reflectometervalue>70) are measured at an angle of incidence or reflection of 20° andmedium glossy surfaces at 60°. A prerequisite for a good gloss ofmetallic coatings or printing inks is likewise a maximum plane-parallelorientation of the platelet-like pigments in the coating medium.

The most brilliant aluminum pigments with the highest gloss and flop areat present assigned to two general classes: on the one hand, theso-called “silver dollar pigments”, which are prepared by wet grindingof aluminum shot, and, on the other hand, the so-called “PVD pigments”.

With PVD pigments, extremely thin Al pigments having mean thicknesses offrom approximately 20 nm to 60 nm are prepared. The thicknessdistribution of these pigments is extremely low. In this method aluminumis vapor-deposited in an ultrahigh vacuum on to a carrier film providedwith a release-coat. This release coat is as a rule a polymer.Subsequently the vapor-deposited aluminum is separated—as far aspossible—from the carrier film in a solvent and the metal film iscomminuted mechanically or ultrasonically. The production of PVDpigments is described, for example, in J. Seubert and A. Fetz, “PVDAluminum Pigments Superior Brilliance for Coatings and Graphic Arts”,Coatings Journal, Vol. 84, A6 225264, July 2001, pages 240-245.

On account of their extreme thinness, these PVD pigments exhibitexcellent covering power. The thin pigments are so flexible that theyvirtually “cling” to the substrate. To display their opticalpossibilities they should therefore be applied to a smooth substrate.

A special effect is achieved in the case of so-called “reverse-sideapplication” in the printing sector. Here, a transparent film is printedwith a printing ink containing the PVD pigments. A metallic lusteralmost resembling a mirror is observed when the film is seen through thenon-printed reverse side after the printing ink has cured. Preferreduses of this application form include headlight reflectors.

The extremely high production costs, however, constitute a drawback ofthese PVD pigments. Another disadvantage is that the release coat canbarely be removed in its entirety from the pigment particles. Thisadherent polymer film can, however, lead to disadvantages. Thus, in thecase of a printing ink, incompatibilities with the solvent used in theprinting ink can occur. For example, polymer films which are suitablefor toluene can be incompatible in solvents such as alcohols or water.This manifests itself in the formation of agglomerates, which completelydestroy the desired decorative effect.

In particular, however, polymeric adhesions of this type can have adeleterious effect when the aluminum pigments, after production thereof,are provided with chemical protective coatings, such as are described,for example, in DE 196 35 085, in order to render themcorrosion-resistant.

The same applies to stabilization by anticorrosive agents, such as aredescribed, for example, in DE 100 01 437. In some circumstances,residues of adherent release-coat can lead to an uneven protectivecoating and hamper the application of a reproducibly produced protectivelayer.

In particular, the use of substrates coated in this way in waterlacquers, in which unstabilized aluminum pigments produce undesirablegassing caused by evolution of hydrogen, is hardly reproduciblyachievable with precoated substrates of this type.

A further serious drawback of PVD pigments is that they exhibit anextremely strong tendency to agglomerate. For this reason, PVD pigmentsare only supplied in highly dilute dispersions having an aluminumpigment content of usually from 10% to 20% by weight. To improvehandling, it is desirable to use compositions having a higher aluminumpigment content.

Silver dollar pigments are distinguished from metal pigments obtained bycomminutive grinding by a relatively round shape and a relatively smoothsurface.

U.S. Pat. No. 4,318,747 discloses fine aluminum effect pigments havingan average size of less than 5 μm with leafing character, which possessa water coverage of at least 50,000 cm²/g and a specific BET surfacearea of from 24 m²/g to 93 m²/g. From these data, roughness values inthe range of from 2.4 to 9.3 can be calculated.

On account of the high degree of roughness of the surface of thesepigments, there is extensive scattering of incident light and aconsequent reduction in gloss, as compared with the smooth surface of aPVD pigment.

EP 1 621 586 A1 discloses aluminum effect pigments obtained by wetgrinding in the thickness range of PVD pigments having an averagethickness of from 25 nm to 80 nm and an average size of from 8 μm to 30μm. However, the drawback of these pigments is that they do not exhibitthe optical properties of PVD pigments.

Finally, EP 1 080 810 B1 relates to an aluminum pigment prepared by wetgrinding of aluminum dust. The aluminum dust, which is also employed asan atomization product for pigment production and which has an averageparticle size of from 2 μm to 10 μm is not described in further detail.

EP 1 424 371 A1 discloses aluminum effect pigments obtained by grindingaluminum powder. According to the teaching of EP 1 424 371 A1, thealuminum powder employed has an average particle size (D₅₀) ranging from1 μm to 10 μm.

The disadvantages of these pigments disclosed in EP 1 621 586 A1, EP 1080 810 B1, and EP 1 424 371 A1 are that they have a very broad relativethickness distribution, which leads to a reduction in quality as regardsthe gloss properties of an article painted or printed with thesepigments.

DE 103 15 775 A1 discloses thin covering aluminum effect pigments havingan average thickness of from 30 nm to 100 nm and a relative thicknessdistribution of from 70% to 140%, which aluminum effect pigments exhibita very high covering power and, on account of their very smooth surface,a gloss similar to that of PVD pigments. In terms of their opticalproperties, they represent a marked improvement over conventional silverdollar pigments with regard to covering power, gloss, and flop. But ithas been seen that the aluminum pigments disclosed in DE 103 15 775 A1sometimes exhibit inadequate transfer behavior, in particular inprinting inks.

It is an object of the present invention to provide very thin aluminumeffect pigments not carrying any adherent polymer film and havingexcellent covering power, high gloss, and an improved metallicappearance as compared with conventional aluminum effect pigments knownfrom the prior art and obtained by conventional wet grinding.

It is a further object of the invention to provide aluminum effectpigments, which additionally exhibit good transfer behavior whenapplying a printing ink containing these aluminum effect pigments. Thealuminum pigments, particularly in printing inks, are intended to comevery close to PVD pigments in respect of their optical properties, butto be significantly easier to produce and handle than the latter. Inparticular, the aluminum pigments are intended to have a markedlyreduced tendency to agglomerate as compared with PVD pigments.

Furthermore, pigments of this type should be capable of being preparedby a more cost-effective method than is the case with the expensiveproduction of aluminum effect pigments using the PVD production method.

This object is achieved by the provision of platelet-like aluminumpigments having

-   a) a mean thickness h₅₀ of from 15 nm to 75 nm as determined by a    scanning electron microscope thickness count,-   b) a relative breadth of thickness distribution Δh of from 30% to    less than 70%, as determined by a scanning electron microscope    thickness count and as calculated on the basis of the corresponding    cumulative breakthrough curve of the relative frequencies of    occurrence, according to the formula Δh=100×(h₉₀−h₁₀)/h₅₀, and-   c) an X-ray diffractogram, measured on pigments in substantially    plane-parallel orientation, having one or two main peaks which do    not correspond to the [111] reflexes.

Preferred developments of the aluminum effect pigments of the inventionare defined in the subclaims 1 to 16.

The object underlying the invention is furthermore achieved by a methodas defined in claim 17 for the production of aluminum effect pigments asdefined in any one of claims 1 to 16, which comprises the followingsteps:

-   a) providing aluminum shot exhibiting a particle size distribution    having a d₁₀<3.0 μm, a d₅₀<5.0 μm, and a d₉₀<8.0 μm,-   b) grinding the aluminum shot defined under a) using a grinder in    the presence of solvent and lubricants and grinding media having an    individual weight of from 2 mg to 13 mg.

Preferred developments of the method are defined in the subclaims 18 to24.

The object of the invention is further achieved by the use of aluminumeffect pigments as defined in claim 25 or claim 26, and a nail varnishcontaining the aluminum effect pigments and defined in claim 27, and aUV-stable printing ink containing the aluminum effect pigments anddefined in claim 28.

The platelet-like aluminum pigments or aluminum effect pigments of theinvention are also referred to hereinafter as “aluminum pigments” forthe sake of simplicity.

The invention relates to platelet-like aluminum pigments having

-   a) a mean thickness h₅₀ of from 15 nm to 75 nm as determined by a    scanning electron microscope thickness count,-   b) a relative breadth of thickness distribution Δh of from 30% to    less than 70%, as determined by a scanning electron microscope    thickness count and as calculated on the basis of the corresponding    cumulative breakthrough curve of the relative frequencies of    occurrence, according to the formula Δh=100×(h₉₀−h₁₀)/h₅₀, and-   c) an X-ray diffractogram, measured on pigments in substantially    plane-parallel orientation, having one or two main peaks which do    not correspond to the [111] reflexes.

The aluminum pigments of the invention possess a very high coveringpower on account of their small mean thickness. The covering power of apigment is usually defined as the area covered per unit weight ofpigment. The smaller the mean thickness of the aluminum pigment, thelarger the area covered by the pigment and consequently the greater thecovering power of the aluminum pigment.

Thin pigments having a narrow thickness distribution advantageouslystack more evenly in the coating medium than conventional aluminumpigments having a broad thickness distribution. With conventionalaluminum pigments, uneven stacking of the pigments can readily occur.Thus, very thick pigments can, in particular, serve as “spacers” andtherefore adversely affect the orientation of the surrounding oradjoining pigments. This adversely affects gloss, flop, and, undercertain circumstances, the covering power. This has a particularlyunfavorable effect on prints. Compared with paint coatings, prints havea substantially smaller thickness and a lower binder content.

The very thin aluminum pigments of the invention having a narrowthickness distribution exhibit, surprisingly, a thickness distributionthat is similar to that of PVD pigments and such pigments are thereforesimilar in their optical properties to PVD pigments, but aresignificantly easier and cheaper to make than PVD-pigments.

It is difficult to determine the exact mean thickness of platelet-likemetal pigments. In practice, the pigment thickness is determined bymeasuring the water coverage (spreading index, DIN 55923) and/or bymeans of a scanning electron microscope (SEM). Only the mean thickness hof the pigments, but not the thickness distribution, can be calculatedfrom the water coverage. The water coverage method fails in the case ofPVD pigments, which show a very marked tendency to agglomerate.Preparation for the spreading test involves drying of the pigments,which leads to irreversible agglomeration phenomena in the case of PVDpigments. Therefore, for the purposes of the present invention, the meanthickness of the aluminum pigments of the invention is determined bymeans of a scanning electron microscope (SEM). Using this method, anadequate number of particles should be measured so as to realize arepresentative statistical evaluation. Customarily, approximately 100particles are measured.

The thickness distribution is advantageously presented in the form of acumulative breakthrough curve. The h₅₀ value of the thickness cumulativebreakthrough curve is taken as a suitable mean value. A measure of thebreadth of distribution Δh, also called span, is given by the followingformula:

$\begin{matrix}{{{\Delta \; h\mspace{14mu} (\%)} = {100 \star \frac{h_{90} - h_{10}}{h_{50}}}},} & (I)\end{matrix}$

in which the indices relate to the respective values of the cumulativebreakthrough distribution.

In the case of the mean thickness h₅₀ of the aluminum pigments of theinvention calculated from the score obtained in the scanning electronmicroscope thickness count (h₅₀ index of the cumulative breakthroughcurve), the mean thickness h₅₀ calculated is from 15 nm to 75 nm,preferably from 18 nm to 70 nm, more preferably from 25 nm to 60 nm andvery preferably from 30 nm to 55 nm.

Below a mean thickness of 15 nm, the pigments become too dark, which canbe attributed to the loss of the metallic reflective capacity whileretaining the high absorption properties of the aluminum. Furthermore,the mechanical properties of the aluminum are changed unfavorably andthe pigments become too brittle. Above a mean thickness of 75 nm, thegood optical properties are increasingly impaired. However, there occursno noticeable impairment up to a mean thickness of 75 nm.

The pigments of the invention preferably possess a relative breadth ofthickness distribution Δh of from 30% to 70%, more preferably from 35%to 67%, still more preferably from 40% to 65% and most preferably from40% to 60%.

Above a Δh of 70%, the advantageous properties of the aluminum pigmentscan no longer be observed. In particular, the high gloss of so-called“reverse-side applications” comparable to that of PVD pigments could nolonger be found. Furthermore, these pigments having a Δh of more than70% sometimes exhibit problems relating to the transfer behavior inprinting applications. Hitherto, it has not been possible to producepigments having a relative breadth of thickness distribution Δh of lessthan 30%.

In another preferred embodiment, the aluminum pigments of the inventionhave a mean thickness h₅₀ of from 25 nm to 60 nm and a span Δh of from35% to 67%. Another particularly preferred embodiment of the aluminumpigments of the invention is characterized by a mean thickness h₅₀ offrom 25 nm to 55 nm and a span Δh of from 35% to 65%.

The aluminum pigments of the invention differ significantly fromconventional PVD pigments in their behavior during X-ray diffractometry.In order to investigate specimens of platelet-like aluminum pigments bymeans of X-ray diffractometry (XRD, X-ray Reflection Diffraction), thepigments are previously oriented so as to be substantiallyplane-parallel relative to the substrate of the specimen. Basically, anycommercially available X-ray diffractometer is suitable for thispurpose.

For the purposes of the invention, a substantially plane-parallelarrangement means that at least 80% of the pigments are parallel to thesubstrate within a tolerance range of +/−15°.

It is established that PVD pigments always have a main peak at thereflex of the [111] plane. By a [111] plane is meant the Miller'sindices. The [111] plane corresponds to the densest possible plane of ametal undergoing face-centered cubic crystallization. This result isknown per se, since it is common knowledge that aluminum sputtered on toa film forms such crystals. However, it has been found, surprisingly,that the aluminum pigments of the invention do not have a main peak atthe reflex of the [111] plane. The reflex of the [111] plane, if at allpresent, is always weak. The main peak or, possibly, main peakspreferentially correspond to the reflexes of the [200] plane and/or the[220] plane. The main peak very preferentially pertains to the [200]plane.

Unlike PVD pigments, the intensity ratio [111]/[200] in the pigments ofthe invention is always<1. This ratio is preferably <0.5 and verypreferably <0.1.

It is presumed that these properties reflect the plastic deformationstate in which the aluminum pigments exist during the grinding process.The at least polycrystalline aluminum shot is exposed to strong shearforces during formative grinding. Shearing occurs between the individualcrystallites, the most densely packed [111] plane obviously being theshear plane. Since the grinding process naturally occurs perpendicularlyto the platelet surface, these planes are broken away from the plateletplane, this being reflected by a drop in the peak intensity in thediffractogram. At the same time, the peaks of the [200] and [220] planesare intensified.

It has been found, very surprisingly, that the aluminum pigments of theinvention have a metallic gloss in “reverse-side applications” which hashitherto not been achieved in conventional aluminum pigments prepared bywet grinding, but has only been possible with PVD pigments.

A reverse-side application is understood to mean that a printing inkpigmented with metal effect pigments is printed on a transparent film.When the cured print is viewed through the unprinted side of the film,in which case an almost mirror-like effect is observed when PVD pigmentsare used. The PVD pigments cling tightly to the film owing to theirsmall thickness and low thickness distribution. Surprisingly, a similareffect can be achieved with the aluminum pigments of the invention. Thesmall overall thickness of the pigments and the small breadth ofthickness distribution are presumably the factors causing this effect.

Furthermore, the determining factor for plane-parallel orientation ofthe pigments is not only surface chemistry incompatibilities of thealuminum pigments and the binder system but also the form factor, whichis another important characteristic determining the properties of theplatelet-like aluminum pigments of the invention.

The form factor f is understood as meaning the ratio of the averagelength to the average thickness of the aluminum pigment platelets.

The length d (diameter) is determined in laser scattering tests on thebasis of the Fraunhofer and/or the Mie diffraction theory. Theevaluation of the diffraction data is based on a model aiming at thediameter of an equivalent sphere. No absolute values are thereforeobtained, but the diameters measured have gained acceptance as reliablerelative values for the description of the size characteristics ofplatelet-like metal pigments.

As regards length, the aluminum pigments of the invention do not differfundamentally from aluminum pigments conventionally available on themarket which have been prepared by wet grinding. Specifically, the sizesdepend on the intended application.

The d₅₀ indices of the length distribution of the pigments arepreferably above 3 μm, more preferably in a range of from 4 μm to 50 μm,even more preferably from 5 μm to 45 μm, still more preferably from 8 μmto 40 μm, very preferably from 10 μm to 30 μm, and most preferably from15 μm to 25 μm.

Furthermore, fine pigments are preferably in the magnitude of from 3 μmto 15 μm and very preferably from 5 μm to 12 μm. Pigments of such typeadditionally preferably exhibit non-leafing properties. They are ground,for example, with oleic acid as the lubricant and are therefore coatedwith this substance. Pigments of this type are especially suitable forreverse-side applications in the printing sector.

The dimensionless form factor f is defined in the present invention as:

$\begin{matrix}{f = {1000 \star {\frac{d_{50}\mspace{14mu} ({µm})}{h_{50}\mspace{14mu} ({nm})}.}}} & ({II})\end{matrix}$

The d₅₀ index of the pigment length corresponds to 50% of the cumulativebreakthrough curve, measured and evaluated in the form of a volumedistribution of equivalent spheres. The mean value h₅₀ of the thicknessdistribution is determined as described above.

The pigments of the invention are distinguished by a form factor f offrom 200 to about 1,500. Preferably, the pigments of the inventionpossess a form factor f of from 210 to 1,000, more preferably from 220to 500 and most preferably from 230 to 400.

A comparatively low content of active aluminum is another characteristicof the pigments of the invention. The content of active aluminum can bedetermined by completely dissolving a defined amount of aluminumpigments in an alkaline solution and recording the resulting hydrogenvolumetrically under temperature-controlled conditions. The activealuminum content of these pigments lies in a range of from 80% to 92%and preferably from 85% to 90%, based on the total weight of thealuminum pigments. These values are below those of conventional aluminumpigments obtained by wet grinding and having a content of activealuminum of from 93% to 97% by weight.

The residual content of non-active aluminum in the pigment can beattributed to aluminum oxide forming naturally on the surface, and tofatty acids bound to the surface. On account of the very low thicknessof the aluminum pigments of the invention, they possess a comparativelyhigh relative oxide content. The content of fatty acids is alsocomparatively high. The latter can be roughly estimated from the Ccontent determined by elemental analysis. In the case of the pigments ofthe invention, the residual content is typically from 0.3% to 1.2% byweight and preferably from 0.4% to 1.0% by weight, as measured onaluminum powders previously washed with acetone or comparable solventsand subsequently dried.

The aluminum pigments of the invention are very thin pigments with avery narrow thickness distribution. Pigments of this type possess a highcovering power. The aluminum pigments of the invention preferablyexhibit a thickness distribution having an h₉₀ index of less than 110nm, preferably less than 100 nm, and more preferably less than 75 nm.Furthermore, the aluminum pigments of the invention have a thicknessdistribution having an h₉₅ index of less than 150 nm, preferably lessthan 120 nm, and more preferably less than 100 nm. The h₉₀ value of thethickness distribution of the aluminum pigments of the invention ispreferably less than 140 nm, more preferably less than 110 nm, and mostpreferably less than 90 nm.

With these very narrow thickness distributions, there are hardly anypigment platelets having a thickness substantially exceeding 100 nm.

The narrow thickness distribution advantageously causes very goodstacking of the aluminum pigments of the invention in a coating medium,for example, a lacquer or a printing ink. With the aluminum pigments ofthe invention, for example, it is possible to obtain lacquers displayinggood coverage and very high gloss and very good flop when applied invery small layer thicknesses, for example, a layer thickness of lessthan 10 μm.

Particularly in the automobile lacquering sector, there is a need forsmall layer thicknesses primarily for cost-saving reasons. Hitherto,basecoat layer thicknesses have been typically in the region of 15 μm.Even now, smaller layer thicknesses are routinely used on very curvedshaped parts, such as door handles. It would be desirable if small layerthicknesses down to less than 10 μm could be realized. However, thelayer thickness should not be too low, as otherwise problems ofadhesion, coverage and/or pigmentation will arise.

In the case of printing inks, the binder contents and the layerthicknesses are generally much lower than in lacquers. This particularlyapplies to gravure printing inks. Gravure printing inks pigmented withconventional aluminum pigments exhibit a solids content of approximately40% by weight. Films printed with gravure printing inks have a wet filmlayer thickness of from about 3 μm to 6 μm and a dry film layerthickness of from about 1.5 μm to 3 μm. In the case of gravure printinginks pigmented with PVD pigments, the solids contents are fromapproximately 15% to 20% by weight of the total gravure printing ink.This is associated with dry film layer thicknesses of from only 0.5 μmto 1.5 μm. In the case of these extremely small layer thicknesses, asubstantially even, plane-parallel orientation of the metal pigments isnecessary, particularly in reverse-side applications. Hitherto, thisorientation could be achieved only when using PVD pigments. The metalpigments of the invention obtained by wet grinding exhibit a similarmean particle thickness and a similar particle thickness distribution.Only those pigments of this type, not hitherto accessible, can exhibitan optical effect in reverse-side application that is comparable to thatobtained when using PVD pigments. Virtually no differences in theoptical quality of the metal pigments of the invention compared with PVDpigments can be observed in gravure applications.

In a further embodiment of the invention, the aluminum pigments of theinvention are subsequently covered or coated with a passivatinginhibitor and/or a passivating anticorrosive layer. Only with coatingsof this type is it possible to safely use the pigments of the inventionin water lacquers and/or in external coatings.

The mechanism of action of the passivating layers is complex. In thecase of inhibitors, it is usually based on steric effects. The majorportion of the inhibitors therefore also has an orienting action asregards leafing or non-leafing, i.e. of being buoyant or non-buoyant inthe coating medium.

The inhibitors are usually added in low concentrations in the order ofmagnitude of from 0.5% by weight to 15% by weight based on the weight ofthe aluminum pigments employed.

Suitable inhibition agents are preferably the following:

-   -   organically modified phosphonic acids or esters thereof of the        general formula

R—P(O)(OR₁)(OR₂),

in which R stands for alkyl, aryl, alkylaryl, arylalkyl, and alkylether, in particular ethoxylated alkyl ether, and R₁, R₂ stand for H,C_(n)H_(2n+1), where n is 1 to 6, in which the alkyl can be branched orunbranched. R₁ and R₂ can be the same or different.

-   -   organically modified phosphoric acids and esters thereof of the        general formula

R—O—P(OR₁)(OR₂),

in which R stands for alkyl, aryl, alkylaryl, arylalkyl, and alkylether, in particular ethoxylated alkyl ether and R₁, R₂ stand for H,C_(n)H_(2n+1), in which n is 1 to 6 and the alkyl can be branched orunbranched.

Pure phosphonic acids or esters thereof or phosphoric acids or estersthereof or any desired mixtures thereof can be used.

In the case of grinding of the aluminum shot in predominantly aqueoussolvents, inhibitors of this type are used as grinding aids in order toprevent the evolution of hydrogen during the grinding process, whichwould constitute a safety hazard.

Furthermore, the passivating inhibitor layer can consist of or includecorrosion-inhibiting organically functionalized silanes, aliphatic orcyclic amines, aliphatic or aromatic nitro compounds, heterocyclicscontaining oxygen, sulfur and/or nitrogen such as, for example, thioureaderivatives, sulfur and/or nitrogen compounds of higher ketones,aldehydes, and alcohols, for example, fatty alcohols, or thiols, ormixtures thereof. The passivating inhibitor layer can, however, alsoconsist of the aforementioned substances. Organic phosphonic acidsand/or phosphoric acid esters or mixtures thereof are preferred. Ifamine compounds are used, these preferably comprise organic radicalshaving more than 6 carbons. Amines of this type are preferably usedtogether with organic phosphonic acids and/or phosphoric acid esters ormixtures thereof.

Passivation by means of anticorrosion barriers having a chemical andphysical protective action can be realized in a variety of ways.

Passivating anticorrosion layers, which guarantee the aluminum pigmentsparticularly good corrosion protection, include or consist of siliconoxide, preferably silicon dioxide, chromium aluminum oxide, which ispreferably applied by a chromating method, zirconium oxide, aluminumoxide, polymerized synthetic resins, phosphate, phosphite or borate, ormixtures thereof.

Silicon dioxide and chromium aluminum oxide layers (chromation) arepreferred. Furthermore, aluminum oxide, aluminum hydroxide or hydratedaluminum oxide layers, such as are described in DE 195 20 312 A1, arepreferred.

The SiO₂ layers are preferably prepared by sol-gel methods with averagelayer thicknesses of from 10 nm to 150 nm and preferably from 15 nm to40 nm, in organic solvents.

In the following, the method for the production of the aluminum pigmentsof the invention will be described. This is distinguished by extremelygentle formative grinding of aluminum shot. Specifically, the methodconsists of the following steps:

-   a) taking aluminum shot having a particle size distribution having a    d₁₀<3.0 μm, a d₅₀<5.0 μm, and a d₉₀<8.0 μm,-   b) grinding the aluminum shot defined under a) using a grinder in    the presence of solvent and lubricants and grinding media having an    individual weight of from 2 mg to 13 mg.

The aluminum shot is preferably prepared in atomizers by atomization ofliquid aluminum, preferably an aluminum melt. The shot includes orconsists of aluminum particles having a preferably approximately roundshape. It is particularly preferred to use aluminum shots havingaluminum particles of a spherical to slightly ellipsoidal shape. Thealuminum shot obtained after atomization of an aluminum melt isclassified, in accordance with a preferred variant, so as to achieve thedesired particle size distribution, which can also be referred to as arange of particle sizes.

The aluminum shot is a very fine metal shot having a very narrow sizedistribution. The range of size distribution is usually determined bylaser diffraction spectrometry, and the particle size can be determinedfrom the laser light diffraction. Laser diffraction spectrometry can becarried out, for example, with the apparatus Helos, supplied by SympatecGmbH, Clausthal-Zellerfeld, Germany, according to manufacturer'sspecifications.

The size distribution has a d_(shot,10)<3.0 μm, a d_(shot,50)<5.0 μm,and a d_(shot,90)<8.0 μm. The size distribution preferably has ad_(shot,10)<0.6 μm, a d_(shot,50)<2.0 μm, and a d_(shot,90)<4.0 μm.

Following the atomization step, the shot can be obtained in the desirednarrow size distribution by means of appropriate classification steps.The classification can be carried out using air classifiers, cyclones,and other known devices.

The aluminum pigments of the invention can be prepared only with the useof such fine and relatively narrow-fraction aluminum shot. As the lowerlimit, the size distribution has the following characteristics:d_(shot,10)>0.15 μm, d_(shot,50)>0.8 μm, and d_(shot,90)>2.0 μm.Consequently, the aluminum shot used comprises predominantly no aluminumshot in nanometric dimensions.

Aluminum shots having d_(shot,50) values ranging from 0.9 μm to 3.0 μmare more preferred and those having values ranging from 0.95 μm to 2.5μm are most preferred.

The aluminum shots preferably used comprise a span of size distribution,which is usually defined asΔd_(shot)=(d_(shot,90)−d_(shot,10))/d_(shot,50), of from 30% to 200% andmore preferably from 40% to 180% and most preferably from 50% to 170%.

The use of such a fine aluminum shot having a narrow size distributionis essential for the production of the metal pigments of the invention.Not all of the aluminum shot particles are transformed evenly during theformative grinding: this means that some particles are transformed to agreater extent whilst some of the shot particles undergo transformationlater in the grinding process. One reason for this is the fact that theprobability of a particle being transformed is dependent on its size.Particles which have already been pre-transformed to form platelets thushave a higher specific surface area than untransformed shot and,accordingly, face a higher probability of being transformed further. Thebreadth of size distribution of the shot is thus taken into account notonly in the size distribution of the aluminum particles formed therefrombut also in the thickness distribution. Therefore, aluminum shot havingappropriately low size variance must be used to obtain narrow thicknessdistributions.

The aluminum shot used for producing the platelet-like aluminum pigmentsof the invention further has a very low oxide content. The content ofaluminum oxide in the aluminum shot is determined by melting thealuminum shot with carbon and determining the resulting carbon monoxideby means of a commercially available apparatus (e.g. Omat 3500 suppliedby JUWE GmbH). The content of aluminum oxide in the aluminum shot isless than 5% by weight, preferably less than 1.5% by weight, and verypreferably less than 1.0% by weight based on the aluminum shot.

In order to achieve these low oxide contents, the atomization step ispreferably carried out in an inert gas atmosphere. Nitrogen and/orhelium are preferably used as inert gases.

The purity of the aluminum used in the atomization process is preferablyfrom 99.0% to more than 99.9% by weight. The shot can contain the usualalloy components (e.g. Mg, Si, Fe) in appropriately small amounts.

The aluminum shot is ground using a grinder, preferably a ball mill, ora stirred ball mill, in the presence of solvent and lubricants acting asgrinding aids, and in the presence of grinding media, which individuallyweigh from 1.2 mg to 13 mg. On account of the extremely gentle manner ofgrinding, this type of grinding takes a comparatively long time. Thegrinding time is preferably from 15 h to 100 h, more preferably from 16h to 80 h and very preferably from 17 h to 70 h.

According to a preferred development of the invention, the grindingmedia individually weigh from 2.0 mg to 12.5 mg and very preferably from5.0 mg to 12.0 mg. The grinding media used are preferably sphericalmedia and more preferably balls.

Balls having a very smooth surface, as round a shape as possible, and ofa substantially uniform size are preferred. The ball material can besteel, glass or ceramics, such as, for example, zirconium oxide orcorundum. The temperatures during the grinding process are in the rangeof from 10° C. to 70° C. Temperatures ranging from 25° C. to 45° C. arepreferred.

Balls made of glass and having a mean individual weight of from 2.0 mgto 12.5 mg are particularly preferred.

Balls made of steel and having a mean individual weight of from 1.2 mgto 4.5 mg, preferably a mean individual weight of from 1.4 to 4.0 mg andmore preferably a mean individual weight of from 2.0 mg to 3.5 mg areused.

The long grinding times lead to a large number of pigment/ball impacts.As a result, the pigment is very uniformly shaped, which is manifestedby a very smooth surface and a very narrow thickness distribution.

With respect to grinding in a ball mill, the critical speed of rotationn_(crit) is an important parameter which indicates when the balls beginto press against the mill wall due to centrifugal forces, at which pointvirtually no more grinding takes place:

$n_{crit} = \sqrt{\frac{g}{2\; \pi^{2}} \star \frac{1}{D}}$

wherein D is the diameter of the drumand g is the gravitational constant.

The speeds of rotation of the ball mill are preferably from 25% to 68%,more preferably from 28% to 60%, and even more preferably from 30% toless than 50% and most preferably from 35% to 45% of the critical speedof rotation n_(crit).

Low speeds of rotation favor slow transformation of the aluminum shot.In order to cause slow transformation, light grinding spheres are alsopreferably used in the method of the invention. Grinding spheresindividually weighing more than 13 mg transform the aluminum shot toovigorously, which leads to premature breakage thereof.

Unlike conventional grinding processes, the aluminum shot in the methodof the invention is for the major part not ground or comminuted, buttransformed extremely gently over a relatively long period of time.

Grinding with very light balls at low speeds of rotation and for a longgrinding time leads to an extremely gentle grinding process, in whichvery thin aluminum pigments are obtained. Since the aluminum shotemployed exhibits a very narrow particle size distribution, the aluminumeffect pigments of the invention also exhibit a very uniform thicknessdistribution.

Grinding can take place in a solvent at a ratio by weight of solvent toaluminum shot of from 2.5 to 10 and at a ratio by weight of the grindingspheres to aluminum shot of from 20 to 110 and using lubricants asgrinding aids.

A large number of compounds can be used as lubricants in the grindingprocess.

In this context, mention may be made of the fatty acids containing alkylradicals having from 10 to 24 carbons which have already been in use formany years. Preferably, stearic acid, oleic acid, or mixtures thereofare used. When stearic acid is used as a lubricant, leafing pigments areformed. Oleic acid, on the other hand, leads to non-leafing pigments.Leafing pigments are characterized in that they are buoyant in a coatingmedium, such as a lacquer or a printing ink, i.e. they float on thesurface of the coating medium. Non-leafing pigments congregate, on theother hand, within the coating medium. Long-chain amino compounds, forexample, can also be added to the fatty acids. The fatty acids can be ofanimal or vegetable origin. Likewise, organic phosphonic acids and/orphosphoric acid esters can be used as lubricants.

The lubricant should not be employed too sparingly, since otherwise thevigorous transformation of the aluminum shot can lead to very largesurface areas of the prepared platelet-like aluminum pigments that areonly inadequately saturated by adsorbed lubricant. In this case, coldwelding occurs. Typical amounts are therefore from 1% to 20% by weight,preferably from 4% to 18% by weight, and very preferably from 8% to 15%by weight of lubricant based on the weight of aluminum employed.

The choice of solvent is not critical as such. It is possible to employcustomary solvents such as white spirit, solvent naphtha, etc. The useof alcohols, such as isopropanol, ethers, ketones, esters, etc. is alsopossible.

Likewise, water (at least as the major portion) can be used as solvent.In this case, the lubricants employed, however, should have a markedanticorrosive action. Phosphonic acids and/or phosphoric acid esters,which can also carry ethoxylated side chains, are preferred. Theaddition of corrosion inhibitors during the grinding process is alsoadvantageous.

Due to the manufacturing method of the invention, the aluminum pigmentsare free from adherent polymer films, which is a great advantage. Thealuminum pigments of the invention therefore do not suffer from thedisadvantages of aluminum pigments still encumbered with residues of therelease coats, such as are prepared by PVD methods. Moreover, theirmanner of production is cheaper than the complicated PVD productionmethods. The separation of the resulting aluminum pigments from thegrinding media, preferably grinding spheres, can be carried out inconventional manner by screening.

After the aluminum shot has been ground, the aluminum pigments obtainedare separated from the grinding media, preferably grinding spheres.

In a further method step, the resulting aluminum pigments can besubjected to size classification. This classification should be carriedout gently, in order not to destroy the thin aluminum pigments. It mayinvolve, for example, wet screening, decantation, or alternativelyseparation by sedimentation caused, for example, by the action ofgravity or by centrifugation. In wet screening, the coarse fraction isusually screened off. In the other methods, the fines, in particular,can be separated. Subsequently, the suspension is freed from excesssolvent, for example, with the aid of a filter press, centrifuge and/orfilter.

In the final step, further processing takes place to give the desireddosage form.

Although the metal pigments of the invention have a similar thicknessand a similar thickness distribution to PVD pigments, they can,surprisingly, be handled more easily. In their dosage forms, thealuminum pigments of the invention are advantageously not restricted toa dilute dispersion form, as is customary in the case of PVD pigments.

Thus the paste form can be used in a manner similar to conventionalaluminum pigments. The solids content is from 30% to 65% by weight,preferably from 40% to 60% by weight, and very preferably from 45% to55% by weight, based on the total weight of the paste.

Furthermore, the aluminum pigments of the invention can be converted toa powder form, preferably a nondusting powder form, by drying. The driedpowder can be further processed to give a nondusting metal powder by theaddition of very small amounts of solvent, for example, from 1% byweight to less than 10% by weight, such as from 3% to 5% by weight,based on the total weight of powder and solvent, in a suitablehomogenizer. Another method is to thoroughly dry the filter cake andthen to retransform it to a paste with another solvent (rewetting).

Surprisingly, the aluminum pigments can alternatively be furtherprocessed by treating the filter cake with a suitable dispersion of asuitable resin to form granules, pellets, briquettes, tablets, or smallcylinders. These dosage forms have the advantage that they do not form adust, are easily metered, and are highly dispersible.

Pelletization can be carried out on a pelletizing plate in conventionalmanner. Tableting can take place in a tableting device. The smallcylinders can be prepared by a molding method for aluminum paste orpowder or by extruding an aluminum paste through an extruder and bychopping the extruded strings of paste by means of a rotating knifesystem. Granulation of the aluminum pigments of the invention can becarried out by, say, spray granulation.

The aluminum pigments of the invention can be provided, to greatadvantage, in the form of granules or pellets having high aluminumpigment contents of, say, from 90% by weight to 35% by weight andpreferably from 70% by weight to 40% by weight.

On account of the very high specific surface area of the aluminumpigments of the invention, relatively large amounts of dispersing resinmust be used, for example, for the process of pelletizing the aluminumpigments of the invention. It is preferred to use from 2% to 50% byweight and more preferably from 5% to 30% by weight of resin, based onthe total formulation of the pellets.

Pelletization may be carried out using a large number of dispersingresins. Examples thereof are both natural and synthetic resins. Theyinclude, for example, alkyd resins, carboxymethyl and carboxyethylcellulose resins, cellulose acetate, cellulose acetate propionate (CAP),and cellulose acetate butyrate (CAB), coumarol-indene resins, epoxideesters, epoxide-melamine, and epoxide-phenol condensates, ethyl andmethyl cellulose, ethylhydroxyethyl cellulose, hydroxyethyl cellulose,hydroxypropylmethyl cellulose, ketonic and maleic acid resins,colophonium resins, melamine resins, nitrocellulose resins, phenolicresins and modified phenolic resins, polyacrylamide resins,polycarbonate resins, polyamide resins, polyester resins, polyetherresins, polyurethane resins, and vinyl resins.

Of these polymeric resins, mention may be made in particular of acrylatecopolymers and acrylic ester resins, polyacrylonitrile resins andacrylonitrile copolymer resins, copolymers of butadiene and vinylidenechloride, butadiene/styrene copolymers, methyl acrylate copolymere andmethyl methacrylate copolymers; and polybutene resins, polyisobutyleneresins, polyvinyl acetate resins, polyvinyl alcohol resins, polyvinylchloride resins, polyvinyl ether resins, polyvinylpyrrolidone resins,and polystyrene resins. Further copolymers include styrene/maleicanhydride resins and styrene/shellac resins, vinyl chloride/vinylacetate resins, vinyl chloride/vinyl ether resins and vinylchloride/vinylidene chloride resins.

Naturally occurring resins such as gum arabic, gutta percha, casein, andgelatin are also suitable.

Aldehyde resins such as the Laropal series produced by BASF AG,Ludwigshafen are preferred. Furthermore, waxes are suitable bindermaterials. Here, natural waxes such as beeswax, candelilla wax, carnaubawax, montan wax, and paraffin wax may be mentioned by way of example.Synthetic waxes such as, for example, PE waxes are likewise suitable.

The aforementioned preparations can be very readily incorporated in,say, lacquer systems or printing inks without the occurrence ofundesirable agglomeration of the aluminum pigments.

It has been found, surprisingly, that the tendency of the aluminumpigments of the invention to agglomerate is much lower than that of PVDpigments.

It is presumed that this effect has to do with the roughness of thealuminum pigments of the invention. The aluminum pigments of theinvention exhibit a certain measure of production-specific roughness orwaviness which prevents plane-parallel adherence, i.e. agglomeration, ofaluminum pigments to one another without, surprisingly, there being anysignificant impairment of the optical properties, such as reflectivecapacity and gloss, of the aluminum pigments of the invention.

Unlike PVD pigments, superposed aluminum pigments of the inventionexhibit, on account of their roughness or waviness, only point-to-pointmutual contact surfaces. As a result—unlike PVD pigments—the formationof short-range forces of attraction, such as van der Waals forces orhydrogen bridges, is minimized and consequently agglomeration oraggregation is hindered.

The aluminum pigments of the invention are used in coatings, lacquers,printing inks, powder lacquers, plastics, and cosmetic formulations.Preferably, the aluminum pigments of the invention are used in printinginks and in nail varnish formulations. The printing inks, nailvarnishes, and coatings of the invention possess a pronounced metallicappearance giving the impression of liquid metal.

The aluminum pigments of the invention are used to particular advantagein printing inks. The aluminum pigments of the invention are used verypreferably in gravure printing inks, screen printing inks, orflexographic printing inks.

The aluminum pigments of the invention passivated by subsequent coatingsare preferably used in water-based lacquers and in exterior coatings.

A further object of the invention is a printing ink containing the metalpigments of the invention, in particular, a liquid printing ink such asgravure printing ink, flexographic printing ink, or screen printing ink.Gravure printing inks, flexographic printing inks, or screen printinginks of this type contain solvents or solvent mixtures. They serve interalia to dissolve the binder, but also to establish important performancecharacteristics of the printing inks, for example the viscosity or thedrying rate.

Solvents used for liquid printing inks, such as flexographic and screenprinting inks, include in particular low-boiling solvents. The boilingpoint is as a rule not more than 140° C. Higher-boiling solvents areemployed only in smaller amounts for adjusting the drying rate. Screenprinting inks are formulated similarly to flexographic or gravureprinting inks but are merely made slightly more viscous and usually havesolvents with a slightly higher boiling point. Examples of suitablesolvents for liquid printing inks include ethanol, 1-propanol and2-propanol, substituted alcohols such as ethoxypropanol, and esters, forexample ethyl acetate, isopropyl acetate, n-propyl acetate and n-butylacetate. It is naturally also possible to use mixtures of varioussolvents. For example, such a mixture may be a mixture of ethanol andesters, such as ethyl acetate or propyl acetate. For printing withflexographic printing plates, it is as a rule advisable for the contentof esters in the total solvent not to exceed about 20% to 25% by weight.Water or a predominantly aqueous solvent mixture is also preferably usedas the solvent for liquid printing inks.

Depending on the type of printing ink, the solvent is used usually in anamount of from 10% to 60% by weight, based on the sum of all components.However, in the case of the printing inks of the invention, a range offrom 60% to 80% by weight of solvent has proved to be particularlyadvantageous.

Radiation-curable printing inks generally do not contain theabovementioned solvents but instead reactive diluents. Reactive diluentstypically perform a dual function. On the one hand, they serve tocrosslink or cure the printing ink. On the other hand, they serve, likeconventional solvents (DE 20 2004 005 921 UI 2004.07.1), to adjust theviscosity. Examples thereof include butyl acrylate, 2-ethylhexylacrylate and in particular polyfunctional acrylates, such as1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate ortrimethylolpropane tri(meth)acrylate.

In principle, binders customary for liquid printing inks can be used asbinders for the metallic printing inks of the invention. A personskilled in the art will make a suitable choice according to the intendeduse and the desired properties. Examples of suitable binders includepolyesters, polyamides, PVC copolymers, aliphatic and aromatic ketoneresins, melamine/urea resins, melamine/formaldehyde resins, maleates,colophonium derivatives, casein and casein derivatives, ethylcellulose,nitrocellulose or aromatic or aliphatic polyurethanes. Polymers orcopolymers of vinyl acetate, vinyl alcohol, acrylates, methacrylates,vinylpyrrolidone or vinyl acetals may also be used. Hyperbranchedpolymers having functional groups, for example hyperbranchedpolyurethanes, polyureas or polyesteramides, can be used to particularadvantage, as disclosed in WO 02/36695 and WO 02/36697. It is naturallyalso possible to use mixtures of different polymeric binders, providedthat the binders chosen do not have any undesired properties whenblended with one another. The amount of all binders is usually from 5%to 40% by weight, based on the sum of all components of the printingink.

Binders particularly preferred include, for example, nitrocellulose,ethylcellulose, hydroxyethyl cellulose, acrylates, polyvinyl butyralsand aliphatic and aromatic polyurethanes and polyureas, in particularhyperbranched polyurethanes and polyureas and mixtures thereof.

Particularly suitable binders for water-dilutable metallic printing inksare copolymers based on (meth)acrylic acid and/or esters thereof withstyrene. Binders of this type are commercially available as solutions ordispersions for use in printing inks, for example under the name Zinpol®(supplied by Worlee). Further examples include aromatic and aliphaticaqueous polyurethanes, polyesters and aqueous polyamides.

Binders preferred for pasty printing inks include, for example,colophonium resins or modified colophonium resins. Examples of modifiedcolophonium resins include those completely or partly esterified withpolyols, for example glycerol or pentaerythritol.

Radiation-curable printing inks comprise binders having crosslinkablegroups, for example olefinic groups, vinyl ether groups or epoxidegroups. Here, the sum of the binders (including reactive diluents) isusually in a range of from 30% to 90% by weight of all components of theprinting ink.

The metallic printing inks of the invention may furthermore comprise oneor more auxiliaries or additives. Examples of additives and auxiliariesare fillers, such as calcium carbonate, hydrated alumina or aluminumsilicate or magnesium silicate. Waxes increase the abrasion resistanceand serve to improve slip. Specific examples thereof are polyethylenewaxes, oxidized polyethylene waxes, petroleum waxes and ceresin waxes.Fatty acid amides can be used to increase the surface smoothness.Plasticizers serve to increase the resilience of the dried film. Forradiation-curable printing inks, at least one photoinitiator or onephotoinitiator system is further used as additive. Dispersants may beused for dispersing the effect pigments. By means of fatty acids, it ispossible to achieve flotation of the effect pigments in the printedlayer such that the pigments accumulate at the top boundary of theprinted layer. Improved metallic effects can advantageously be achievedthereby. Furthermore, antisedimentation agents may also be added. Suchadditives prevent sedimentation of the effect pigments. Examples includesilica, cellulose derivatives and waxes.

The addition of antisedimentation agents is generally advisable,although not always absolutely essential, when formulating theparticularly preferred low-viscosity flexographic, gravure or screenprinting inks. The total amount of all additives and auxiliaries shouldusually not exceed 20% by weight, based on the sum of all components ofthe printing ink, and is preferably from 0.1% to 10% by weight.

The preparation of the metallic printing inks of the invention can becarried out in a manner basically known by thorough mixing or dispersionof the components in conventional apparatus, for example dissolvingtanks or stirred tanks. When dissolving tanks are used, a person skilledin the art will ensure that the energy input is not too high, in orderto avoid damaging the metal effect pigments. Conversely, it mustnaturally be sufficiently high to permit proper dispersion of thepigments. If, in addition to the metal effect pigments of the invention,conventional colored pigments are also used, it may be advisable topredisperse them in a portion or all of the solvent, binder and anyauxiliaries used in the metallic printing ink and to add the metaleffect pigments of the invention at a later stage. In this way,particularly good dispersion of the additional pigments is achievedwithout damaging the metal effect pigments by unduly high dispersingforces. Instead of the pigments, predispersed pigment concentrates maybe added. In a particularly elegant procedure, it is possible, in thiscase, to use a commercial printing ink in small quantities, providedthat the added printing ink is compatible with the formulation of themetallic printing ink and does not adversely affect the propertiesthereof.

The following examples serve to provide a non-restrictive explanation ofthe invention.

EXAMPLE 1 a) Atomization

Aluminum bars are continuously fed to an induction crucible furnace(supplied by Induga, furnace capacity approximately 2.5 t) and meltedtherein. The aluminum melt is present in the so-called forehearth in aliquid state at a temperature of approximately 720° C. A number ofinjector-type nozzles dipping into the melt atomize the aluminum meltvertically upwardly. The motive gas is compressed in compressors(supplied by Kaeser) to up to 20 bar and heated in gas heaters to about700° C. The resulting aluminum shot solidifies and cools down in flight.The induction crucible furnace is integrated in a closed plant.Atomization is carried out under inert gas (nitrogen). The aluminum shotis initially precipitated in a cyclone, and the pulverulent aluminumshot precipitated here has a d₅₀ of from 14 μm to 17 μm. A multicycloneserves to effect further precipitation, and the pulverulent aluminumshot precipitated in this multicyclone has a d₅₀ of from 2.3 μm to 2.8μm. The gas/solids separation occurs in a filter (supplied by Alpine)having metal elements (supplied by Pall). Aluminum shot having a d₁₀ of0.7 μm, a d₅₀ of 1.9 μm, and a d₉₀ of 3.8 μm is obtained as fines.

b) Grinding

4 kg of glass spheres (diameter: 2 mm), 75 g of finest aluminum shotobtained under a), 200 g of white spirit, and 3.75 g of oleic acid areplaced in a barrel mill (length 32 cm, breadth 19 cm). The mixture isthen ground at 58 rpm for a period of 15 h. The product is separatedfrom the grinding balls by rinsing with white spirit and subsequentlyscreened in a wet screening process on a 25 μm screen. The fines aresubstantially freed from white spirit by means of a suction filter andsubsequently worked to a paste with white spirit in a laboratory mixer(approx. 50% solids).

EXAMPLE 2 a) Aluminum Shot

Aluminum shot produced in accordance with Example 1 was used. The shotexhibits the following characteristics of its size distribution curve:

d_(10,shot)=0.7 μm; d_(50,shot)=1.6 μm; d_(90,shot)=3.2 μm.

b) Grinding

4.7 kg of glass spheres (diameter: 2.0 mm), 67 g of finest aluminum shotobtained under 2a, 200 g of white spirit, and 10 g of oleic acid areplaced in a barrel mill (length: 32 cm, breadth: 19 cm). The mixture isthen ground at 43 rpm for a period of 22 h. The product is separatedfrom the grinding balls by rinsing with white spirit and is subsequentlyscreened in a wet screening process on a 25 μm screen. The fines aresubstantially freed from white spirit by means of a suction filter andsubsequently worked to a paste with white spirit in a laboratory mixer(approx. 50% solids).

COMPARATIVE EXAMPLE 3

Commercially available Metalure L-55350 (Eckart)

COMPARATIVE EXAMPLE 4

Commercially available Silvershine S 2100 (Eckart), prepared asDescribed in DE 103 15 775

COMPARATIVE EXAMPLE 5

Commercially available VP 53534 (Eckart), silver dollar pigment forlacquer.

COMPARATIVE EXAMPLE 6

Commercially available RotoVario 530 080 (Eckart), silver dollar pigmentfor gravure printing.

COMPARATIVE EXAMPLE 7

Commercially available RotoVario 500 042 (Eckart), silver dollar pigmentfor gravure printing.

The specimens of Examples 1 and 2 of the invention and ComparativeExamples 3 to 5 were characterized for a closer determination of theparticle thicknesses using a field ion scanning electron microscope.

The specimens were prepared for determining the thickness distributionby means of an SEM as described below:

a) Aluminum Pigments of the Invention and Conventional Pigments Obtainedby Wet Grinding EXAMPLES 1 AND 2 AND COMPARATIVE EXAMPLES 4 AND 5

The aluminum pigments of the invention and the conventional aluminumpigments obtained by conventional wet grinding are in each case presentin the form of a paste or filter cake and are each first washed withacetone and then dried.

A resin customarily used in electron microscopy, for example TEMPFIX(Gerhard Neubauer Chemikalien, D-48031 Munster, Germany), is applied toa specimen plate and heated to softening temperature on a hotplate.Subsequently, the specimen plate is taken from the hotplate and therespective aluminum powder is scattered onto the softened resin. Theresin resolidifies on cooling and the scattered aluminum pigments—due tothe interplay between adhesion and the force of gravity—can be preparedextending almost vertically and fixed to the specimen plate. As aresult, the pigments can be easily measured laterally in the electronmicroscope. When measuring the thickness, the azimuthal angle α of thepigment is estimated relative to a plane normal to the surface and isallowed for when evaluating the thickness according to the formula

h _(eff) =h _(mess)/cos α

The cumulative breakthrough curve was plotted from the h_(eff) valuesagainst the relative frequencies of occurrence. In all cases, 100particles were counted.

b) PVD Pigments COMPARATIVE EXAMPLE 3

A PVD pigment suspension was washed a number of times with a largeexcess of acetone in order to substantially free it from residues of therelease coat. Subsequently, the PVD pigments were dispersed in acetoneand a drop of the dispersion was distributed on a microscope slide.Following evaporation of the solvent, the slide was sliced. Theindividual slices can be vertically mounted in the electron microscope.At the sharp broken edges, sufficient PVD pigments can be measured.Again, 100 particles were counted.

The cumulative breakthrough curves of the thickness distribution of theexample of the invention and Comparison Examples 3 to 5 are shown inFIG. 1. Statistical analyses showed that the cumulative breakthroughcurve was substantially constant over from 75 to 100 particles for thepigments of the invention and conventional pigments obtained by wetgrinding.

The d₁₀, d₅₀, and d₉₀ values and the span values of the sizedistribution (Cilas) calculated therefrom, the correspondingcharacteristics of the thickness measurement from SEM investigations,the span diameter/thicknesses calculated therefrom, the form factors fof the pigments, and the active metal contents are listen in Table 1below.

The length d was determined with the aid of a laser granulometer (Cilas1064, Cilas, France) and the d₅₀ index of the cumulative breakthroughdistribution in μm was chosen as a measure of the mean length in theusual manner.

The pigments of Example 1 and Comparative Example 3 were subjected to anX-ray diffractometric investigation. The readings were taken using apowder diffractometer supplied by Thermoelektron (produced in Ecublens,Switzerland) of the type X'tron. A copper tube was used as the X-raysource and the Cu—K_(α1,2) line was used for excitation. The device hada Bragg-Brentano measuring geometry.

For specimen preparation, the pigments were washed with acetone. Then afew drops of the pigment/acetone dispersion were applied to a rotaryplate and dried at room temperature. The pigments thus orient themselvessubstantially parallel to the substrate.

The corresponding diffractograms are shown in FIGS. 2 and 3. Theintensity of the X-rays measured is shown as a function of the measuringangle. Lines mark the positions of the reflexes of specificcrystallographic planes as are to be expected according to theICDD-(International Center for Diffraction Data).

The measuring period for recording a diffractogram was several hours. Itcan be clearly seen that the spectrum of Comparative Example 3 (PVDpigments, FIG. 3) exhibits appreciable peaks only in the [111] planesand [222] planes. The reflex of the [222] plane is the higher order ofthe [111] plane and is much weaker. These findings point to a largelymonocrystalline structure of the pigment, with the densely packed [111]plane being present parallel to the platelet surface.

As can be seen from FIG. 2, the X-ray diffractogram of the example ofthe invention has a very pronounced main peak at the [200] plane.Furthermore, signals corresponding to the [220] plane and—in a muchweaker manner—to the [111] and the [311] planes can be seen. Theintensity of all signals is clearly weaker than in the case of the PVDpigment in spite of a longer integration time. Accordingly, thecrystalline character of this pigment obtained by wet grinding is, onthe whole, much weaker. These reflexes are characteristic of an aluminumpigment in a state of plastic deformation and therefore reflect thephysical state of an aluminum pigment during or after grinding.

TABLE 1 Physical characteristics Particle thickness distribution ActiveLength Span from SEM Span metal D₁₀ d₅₀ d₉₀ Length h₁₀ h₅₀ h₉₀ ThicknessForm content Specimen [μm] [(d₉₀ − d₁₀)/d₅₀] [nm] [(h₉₀ − h₁₀)/h₅₀]factor [%] Example 1 7.1 13.0 20.0 0.99 35 54 70.2 0.67 245 87 Example 25.1 9.2 14.2 0.99 22 32 43 0.65 288 81 Comp. 5.7 12.7 20.7 1.18 38 47 520.31 270 — Example 3 Comp. 11.3 20.1 32.4 1.05 46 74 145 1.36 272 95Example 4 Comp. 9.8 17.2 26.9 0.99 55 137 330 2.01 126 97 Example 5Comp. 6.0 11.5 20.5 1.26 159 — 72 92 Example 6 Comp. 7.3 15.7 26.6 1.23— — — 96 Example 7

The following Table 2 lists the calorimetric data of wet lacquercoatings of selected examples.

The reverse-side applications in Table 2 were prepared using a gravureprinting ink based on a commercially available polyvinyl butyral byimprinting a MELINEX 400 film (PET film, 50 μm) firstly by means of adoctor blade having a blade gap of 24 μm and secondly by means of aprinting machine (printing machine: Rotova 300, supplied by Rotocolor, 3inking units; printing speed 100 m/min, viscosity 15 s DIN 4 flow cup;70 lines/cm; pigmentation level depending on particle thickness between3.5% (Example 3) and 14.5% (Example 7).

The reverse-side applications were characterized optically by glossmeasurement at 600 following DIN 67 530 (apparatus: micro-TRI glossmanufactured by Byk-Gardner, D-82538 Geretsried, Germany). The apparatuswas calibrated for this purpose by means of dark calibration and a blackmirror glass plate with values of 92 at 60°.

The evaluation of the gloss measurement carried out at 60° in theconventional manner shows that the pigments prepared according toExamples 1 and 2 of the invention exhibit a far higher gloss thanconventional pigments obtained by conventional wet grinding (seeComparative Examples 6 and 7).

The visual impression of the pigments prepared according to Examples 1and 2 of the invention is also distinguished by a very intenselymetallic mirror effect—similar to that observed in PVD pigments (seeComparative Example 3).

The gloss of the pigments of the invention in this applicationapproximately corresponds to that of PVD pigments (see ComparativeExample 3).

In Comparative Examples 4 and 5, the reverse-side application could notbe carried out to satisfaction by means of gravure printing. Thepigments exhibited a transfer behavior that was inadequate in thegravure printing process owing to their particle size. Reverse-sideapplications could only be achieved by means of a doctor blade. Thedoctor blade technique, however, is not applicable for commercialpurposes in most cases. In the case of Comparative Example 4, a highgloss and metallic effect were likewise achieved with the applicationsmade by doctor blade, but no mirror effect.

The conventional silver dollar pigments for gravure printing representedby Comparative Examples 5 and 6 exhibit much less gloss and by no meansapproach the mirror effect pertaining to Examples 1 and 2 or ComparativeExample 3.

The results of the calorimetric measurements prove that, on account oftheir small thickness and low thickness distribution, the pigments ofthe invention show, similarly to PVD pigments, extremely goodorientation and thereby very high direct reflection, i.e. a high degreeof gloss measured at 60°.

TABLE 2 Reverse-side application on MELINEX film Reverse-sideapplication by Reverse-side application by doctor printing blade GlossVisual Gloss Visual Specimen 60° impression 60° impression Example 1 of643 very metallic, “mirror effect” 619 Very metallic “mirror effect” theinvention Example 2 of 660 very metallic, “mirror effect” 650 verymetallic, “mirror effect” the invention Comp. 677 very metallic “mirroreffect” 672 very metallic “mirror effect” Example 2 Comp. — — 504metallic Example 3* Comp. — — 366 metallic, white Example 4* Comp. 494metallic 445 metallic Example 5 Comp. 339 metallic, white 267 metallic,white Example 6 *These specimens could not be applied since the transferbehavior was inadequate.

Examples concerning the passivation of the aluminum pigments of theinvention:

EXAMPLE 8 (SiO₂-Coated Aluminum)

55.1 g of a paste containing aluminum pigments as described in Example 1(equivalent to 38.5 g of Al) were dispersed in 375 ml of isopropanol andbrought to the boil. 13.35 g of tetraethoxysilane were added.Subsequently, a solution of 5.4 g of 25% strength NH₃ in 9.3 g of waterwas metered in over a period of 3 h. After a further 3 h, the mixturewas cooled to room temperature and the suspension was filtered off withsuction by means of a Buchner funnel. Subsequently, the product wasdried overnight in a vacuum drying oven at 100° C.

EXAMPLE 9 Chromated Aluminum

18 g of a chromic acid solution were prepared by dissolving 4.5 g ofCrO₃ in 13.5 g of demineralized water.

220 g of demineralized water were heated to 90° C. in a reactor having acapacity of 1 liter. With vigorous stirring (stirring unit:Stollenscheibe), first 21 g of butyl glycol were added and then 125 g ofthe aluminum pigments described in Example 1 in the form of a whitespirit paste having a solids content of 70%. A few minutes later, thechromic acid solution was added at a reaction temperature of 80° C. Themixture was allowed to react for a further 50 min with vigorousstirring. The reaction mixture was then allowed to cool for 30 min anddecanted into a beaker a number of times with 250 mL of a 5% strengthdemineralized H₂O/butyl glycol solution each time until yellowcoloration of the supernatant solution no longer occurred. The productwas then filtered off in a suction filter and washed with approx. 3liters of water.

Gassing Test

8.6 g of Al were incorporated in the form of a paste into 315 g of acommercially available colorless water-based lacquer and adjusted to pH8.2 with dimethanol-ethanolamine. 300 g of this lacquer were filled intoa gas wash bottle and this was closed with a double-chamber gas bubblecounter. The amount of gas could be determined from the amount of waterdisplaced in the lower chamber of the gas bubble counter. The gas washbottle was temperature-controlled at 40° C. in a water bath and the testwas carried out over a period of 30 days. The specimen was regarded ashaving passed the test if after 7 days not more than 4 mL, and after 30days not more than 20 mL, of hydrogen had evolved.

TABLE 3 Results of gassing test on coated thin aluminum pigmentsSpecimen Gassing for 7 d Gassing for 30 d Example 8 1 mL 7 mL Example 91 mL 4 mL Comp. example <3 h !! — (uncoated pigment of Example 1) d:days

It can be seen from Table 3 that the aluminum pigments of the inventioncan be extremely well stabilized against corrosion.

EXAMPLE 10 Nail Varnish

Aluminum shot was atomized and then ground as in Example 1. Fatty acidsof vegetable origin were used as lubricants. The pigment was washed fivetimes with isopropanol by means of a Buchner funnel in order to removeresidues of white spirit originating from the wet grinding process.

Subsequently, nail varnishes of the following composition were preparedfrom this pigment and from Comparative Example 3:

TABLE 4 Nail varnish formulations: No. Substance Example 10 ComparativeExample 11 1 Metal pigment of Metalure ® Example 10 CA-41010 AE* 19% byweight 2 Methyl ethyl ketone 21% by weight 21% by weight 3 Methylisobutyl 20% by weight 20% by weight ketone 4 CAB 381.2  8% by weight 8% by weight 5 Butyl acetate 32% by weight 32% by weight 98/100*Commercially available PVD pigments for cosmetic purposes (supplied byEckart)

The nail varnishes in accordance with Example 10 and Comparative Example11 were applied to an artificial fingernail. Both applications showed ahighly glossy, silvery, continuous metal film resembling a “liquidmetal”. Comparative Example 11, however, exhibited slightly highergloss.

The invention relates to aluminum pigments, which in their physicalproperties come very close to the PVD pigments, but can be prepared in asignificantly simpler manner. Finally, the aluminum pigments of theinvention do not show any tendency to agglomerate, which is a drawbackof PVD pigments. Compared with conventional aluminum pigments, thealuminum pigments of the invention exhibit very improved properties, inparticular as regards their covering power and their gloss. Inparticular, in reverse-side applications in the printing sector, thealuminum pigments of the invention exhibit properties that arecomparable to those of PVD pigments. This had hitherto not been achievedby pigments obtained by wet grinding.

1. Platelet-like aluminum pigments having a narrow thicknessdistribution and at least partially coated with lubricant, wherein thepigments have a) a mean thickness h₅₀ of from 15 nm to 75 nm asdetermined by a scanning electron microscope thickness count, b) arelative breadth of thickness distribution Δh of from 30% to less than70%, as determined by a scanning electron microscope thickness count andas calculated on the basis of the corresponding cumulative breakthroughcurve of the relative frequencies of occurrence, according to theformula Δh=100×(h₉₀−h₁₀)/h₅₀, and c) an X-ray diffractogram, measured onpigments in substantially plane-parallel orientation, having one or twomain peaks which do not correspond to the [111] reflexes.
 2. Theplatelet-type aluminum pigments according to claim 1, wherein saidaluminum pigments are produced by grinding processes.
 3. Theplatelet-type aluminum pigments according to claim 1, wherein saidaluminum pigments exhibit a relative breadth of said thicknessdistributions Δh of from 35% to 65%.
 4. The platelet-type aluminumpigments as defined in claim 1, wherein the aluminum pigments have aform factor d₅₀/h₅₀ of from 200 to
 1500. 5. The platelet-type aluminumpigments as defined in claim 1, wherein the X-ray diffractogram has oneor two peaks of maximum intensity assignable to at least one of the[200] and [220] reflexes.
 6. The platelet-type aluminum pigments asdefined in claim 1, wherein the aluminum pigments are coated at leastpartially with fatty acids as lubricants.
 7. The platelet-type aluminumpigments as defined in claim 1, wherein the aluminum pigments are coatedat least partially with stearic acid as lubricant.
 8. The platelet-typealuminum pigments as defined in claim 1, wherein said aluminum pigmentsare coated at least partially with oleic acid as lubricant.
 9. Theplatelet-type aluminum pigments as defined in claim 1, wherein saidaluminum pigments are coated at least partially with a mixture ofstearic acid and oleic acid as lubricants.
 10. The platelet-typealuminum pigments as defined in claim 1, wherein said aluminum pigmentsare at least partially coated with phosphonic acids, phosphates, or amixture thereof as lubricants.
 11. The platelet-type aluminum pigmentsas defined in claim 1, wherein the aluminum pigments are coated with apassivating inhibitory layer or anticorrosive layer.
 12. Theplatelet-type aluminum pigments as defined in claim 11, wherein saidpassivating inhibitory layer comprises at least one selected from thegroup consisting of anticorrosive organic phosphonic acids andphosphoric acid esters, organically functionalized silanes, aliphaticand cyclic amines, aliphatic and aromatic nitro compounds, heterocycliccompounds containing at least one heterocyclic atom selected from thegroup consisting of oxygen, sulfur and nitrogen, and sulfur and nitrogencompounds of higher ketones, aldehydes, and alcohols, thiols, andmixtures thereof.
 13. The platelet-type aluminum pigments as defined inclaim 11, wherein said passivating anti-corrosive layer comprises atleast one selected from the group consisting of silicon dioxide,zirconium oxide, aluminum oxide, chromium oxide, polymerized syntheticresins, vanadium oxides, molybdenum oxides and peroxides, phosphates,phosphates, borates, and mixtures and combinations thereof.
 14. Theplatelet-type aluminum pigments as defined in claim 13, wherein saidpassivating anti-corrosive layer comprises silicon dioxide.
 15. Theplatelet-type aluminum pigments as defined in claim 1, wherein thealuminum pigments are oxidized by water in a chemical wet process andthe aluminum pigments have a colored appearance.
 16. The platelet-typealuminum pigments as defined in claim 1, wherein the aluminum pigmentsexist as powders.
 17. A method for the production of aluminum effectpigments as defined in claim 1, wherein the method comprises thefollowing steps: a) providing aluminum shot exhibiting a particle sizedistribution having a d_(shot,10)<3.0 μm, a d_(shot,50)<5.0 μm, and ad_(shot,90)<8.0 μm, b) grinding the aluminum shot defined under a) usinga grinder in the presence of solvent and lubricants and grinding mediahaving an individual weight of from 1.2 mg to 13 mg.
 18. The method asdefined in claim 17, wherein said grinding media have an individualweight of from 5.0 mg to 12 mg.
 19. The method as defined in claim 17,wherein said aluminum shot as produced according to step a) has aparticle size distribution having a d_(shot,10)<0.6 μm, ad_(shot,50)<2.0 μm, and a d_(shot,90)<4.0 μm
 20. The method as definedin claim 17, wherein the grinding time is from 15 to 100 hours.
 21. Themethod as defined in claim 17, wherein said aluminum pigments aresubjected in a further step (c) to size classification.
 22. The methodas defined in claim 17, wherein the aluminum pigments provided in saidstep b) are converted to a compact form.
 23. The method as defined inclaim 17, wherein the aluminum pigments provided in said step b) areconverted to an aluminum powder.
 24. The method as defined in claim 17,wherein the solvents used are organic solvents.
 25. The method asdefined in claim 17, wherein the solvent used is water and thelubricants used are at least one selected from the group consisting oforganic phosphonic acids and esters thereof and phosphoric acids andesters thereof. 26-29. (canceled)
 30. A cosmetic formulation, whereinsaid cosmetic formulation contains aluminum pigments as defined inclaim
 1. 31. A printing ink, wherein said printing ink contains aluminumpigments as defined in claim
 1. 32. The cosmetic formulation of claim30, wherein the formulation is a nail varnish.
 33. A water-basedlacquer, wherein said lacquer contains coated aluminum pigments asdefined in claim
 11. 34. A coating composition adapted for exteriorapplication, wherein said coating composition contains coated aluminiumpigments as defined in claim
 11. 35. A method for producing acomposition selected from the group consisting of coatings, lacquers,printing inks, powder-based ceramics and cosmetic formulations, saidmethod comprising incorporating aluminum pigments as defined in claim 1in said composition.
 36. The platelet-type aluminum pigments defined inclaim 14, wherein a surface of the silicon dioxide layer is coated withsilanes.
 37. The platelet-type aluminum pigments as defined in claim 16,wherein the powder is a non-dusting powder.
 38. The platelet-typealuminum pigments as defined in claim 16, wherein the powder is in apaste or in a compacted form selected from the group consisting ofgranules, pellets, tablets, small cylinders and briquets.
 39. The methodas defined in claim 22, which further comprises selecting the compactform from the group consisting of paste, granules, tablets, smallcylinders, briquets and pellets.
 40. The method as defined in claim 23wherein the aluminum powder is a non-dusting aluminum powder.
 41. Themethod as defined in claim 24, which further comprises selecting theorganic solvent from the group consisting of white spirit, solventnaphtha, isopropanol, alcohols, ketones and mixtures thereof.
 42. Amethod for producing a reverse-side application in at least one selectedfrom the group consisting of gravure printing, flexographic printing andscreen printing, the method comprising printing on a transparent filmwith a printing ink comprising aluminum pigments according to claim 1.